1. Field
The embodiments discussed herein relate to broadband wireless telecommunication networks, and in particular to a method for a preventing ping pong handover effect in mobile WiMAX networks. References indicated with and used acronyms are reported at the end of the description.
More particularly, the embodiments apply to a Mobile Wireless Broadband Access network conforming to IEEE std 802.16-2004 as amended by IEEE 802.16e-2005 and corrected by IEEE 802.16-2004 Corrigendum 2. The embodiments can be extended also to Mobile Wireless Broadband Access network conforming to IEEE 802.16 Rev2. The present embodiments apply as well to the WiMAX Forum specifications (i.e. Stage 2 and Stage 3 specifications).
2. Description of the Related Art
As reported in the WiMAX Forum, Mobile WiMAX is a broadband wireless solution that enables convergence of mobile and fixed broadband networks through a common wide area broadband radio access technology and flexible network architecture. The Mobile WiMAX Air Interface adopts Orthogonal Frequency Division Multiple Access (OFDMA) for improved multi-path performance in non-line-of-sight environments. Scalable OFDMA (SOFDMA) is introduced in the IEEE 802.16e Amendment to support scalable channel bandwidths from 1.25 to 20 MHz.
FIG. 1, that will be discussed later on, shows the reference architecture for a WiMAX network. In its essentiality, we see a mobile station (MS) wirelessly connected to a base station BS1 through R1 interface. The MS, because of various causes, can request the network to handover the user's connection from an actual radio channel to another of a different cell (or the same cell). The BS can trigger a handover procedure due to several reasons, either in reply to a MS request or autonomously (e.g. for radio channel quality, traffic load conditions, resource status). Some triggering causes require immediate handover execution (imperative handover), otherwise call drops or severe interference can occur. Other triggering causes do not require immediate execution but are intended for optimisation purposes only: e.g. for individual radio link optimisation (power budget handover) or for network resource optimisation (handover for load balancing criteria, etc). In the second case the target BS may immediately trigger handover back to the previous serving BS because the previous serving BS may provide better radio channel conditions. Such a back and forth can be iterated many times before it is stopped (generally at the expiration of a counting), giving rise to the so called “Ping-pong” effect that may cause system instability, a call drop increase, and QoS degradation due to unnecessary increase of handover events
At the present stage of WiMAX standardization, even when looking for the nearest cellular technologies, the teaching is always to inhibit ping-pong handovers independently of their typology. Additional interference is introduced by this way of proceeding, as also those handovers that should be useful to do will be prevented, i.e. power budget handovers repeatedly commanded between the two BSs because the mobile is really going back and forth between the two cells.
In addition, WiMAX, GSM, and W CDMA, in order to reduce system instabilities, adopt hysteresis margins for handover. An effect of hysteresis margins is to prevent the MS from connecting immediately with the most suitable cell, reducing, in this way, the ping pong effect. The larger the hysteresis, the lower the system performance, as the MS remains for a longer time connected with a suboptimal cell. Large hysteresis values are actually particularly harmful in low frequency reuse systems, such as WiMAX, due to additional interference generated on close neighbouring cells.